After a year at sea, a sailor returns to his home port and walks into his favorite bar, and everyone turns to stare at him because his head has shrunk to the size of a grapefruit. Finally, one of his oldest friends asks him what has happened. And the sailor tells this story:
"We were at sea, and it was fine weather with a fair wind, and there wasn't much to do that day, so I decided to do a little fishing. I felt this immense tug on the line, and when I reeled in my catch, what had I caught but the most beautiful mermaid in all the seven seas!
"And she said to me, 'Mr. Sailor, sir, please, won't you let me go! I am a magical mermaid, and I can grant you your very fondest wish if only you'll release me.'
"And so I said to her, 'Well, Miss Mermaid, ever since I went to sea, I've had only one dream: to make love to a mermaid. So if we can go below...'
"But she interrupted me, and said, 'Alas, Mr. Sailor, I'm sorry, but that's the one wish I can't grant, because as you see, I'm a woman from the waist up, but I'm a fish from the waist down.'
"And so I said to her, 'Well, that's OK, Miss Mermaid. Why don't you just give me a little head?'"
Via Andrew Sullivan.